Effects of Oxide Precursors on the Structure and Properties of Na₃PS_4–<i>x</i>O_<i>x</i> Glassy Solid Electrolytes

Solid-state sodium batteries based on sulfur cathodes offer the advantage of both sustainability due to the abundance of sodium and sulfur and the safety offered by solid electrolytes. However, they are hampered by the challenges associated with the sodium solid-state electrolytes, sodium–metal anodes, and sulfur cathode. We present here the synthesis of Na3PS4–xOx glassy solid electrolytes (GSEs) through a facile mechanochemical reaction of various oxide precursors and milling conditions, aiming to identify the relationship between the stability of the oxide precursor and the extent of oxygen integration. Structural motifs unique to each oxygen precursor are identified with 23Na and 31P magic-angle spinning nuclear magnetic resonance and Raman spectroscopies to identify anion mixing and oxygen incorporation. Additionally, the degree of unreacted Na2S during the synthesis is quantified and correlated with the transport properties. The cycling stabilities of solid-state Na–S cells assembled with Na3PS4 and Na3PS3.8O0.2 electrolytes are compared at room temperature. Notably, cells assembled with the Na3PS4–xOx GSE that was prepared with P2O5 exhibit 980 mA h g–1 after 50 cycles at 25 °C with an areal capacity of 0.5 mA h cm–2. This study demonstrates the potential of safer, sustainable solid-state batteries with earth-abundant sodium and sulfur.